BCNU, an alkylating antitumor drug, is a potent inhibitor of GSSG reductase in liver and red blood cells. Since GSSG reductase is responsible for maintaining the intracellular antioxidant defense mechanisms, inhibition of this enzyme by BCNU may ultimately lead or contribute to pulmonary damage. Single doses of BCNU inhibited lung GSSG reductase in a dose- and time-dependent manner. The depression of reductase activity was persistent, lasting up to 8 days after BCNU administration. The multi-dosing BCNU treatment regimen produces a delayed onset interstitial fibrosis in the lung. This treatment regimen also causes a 70% reduction of pulmonary GSSG reductase and a 300% increase in pulmonary GSSG levels. These effects were specific for the lung; there was no marked or persistent inhibition of GSSG reductase or alteration of GSH/GSSG ratio in kidney, liver or heart tissue. The inhibition of pulmonary GSSG reductase by BCNU preceded the onset of marked pulmonary damage. BCNU-induced inactivation of lung GSSG reductase occurred in vitro, required NADPH, and was time- and concentration-dependent. Exogenously added substrate or sulfhydryl compounds were capable of decreasing the amount of enzyme inactivated by BCNU. The distribution of 14C-BCNU in F344 rats demonstrated that BCNU was not preferentially accumulated in pulmonary tissue either as total 14C (parent drug and metabolites) or as radioactivity covalently bound to cellular macromolecules. These studies demonstrate that there is a preferential destruction of lung GSSG reductase by BCNU which precedes the development of severe lung toxicity and that this preferential BCNU-induced lung toxicity is not due to a preferential accumulation in lung tissue.